The present invention relates to a varifocal camera, and more particularly, to a small-sized varifocal camera having a high variable power zooming function.
Recently, cameras equipped with zoom lenses are in demand, even in the field of cameras of a lens shutter type, and the main current in the field of multi-focus cameras is changing from dual-focus types to zoom lens types. The range of focal distances of a zoom lens to be equipped on a camera of a lens shutter type in the early stages, was limited to cover only a wide angle lens and a semi-telephoto lens, both of which make a variable power ratio of about two. With the spread of zoom lenses in the market, however, the demand for a higher variable power ratio has increased.
From the viewpoint of the structure of a zoom lens with a high variable power ratio to be used in a camera of a lens shutter type, a zoom lens of a two-group type is not suitable because of difficult aberration correction and a longer distance of lens movement. As a zoom lens with a high variable power ratio, therefore, there is known a zoom lens of a four-group type, for example, disclosed in Japanese Patent Publication Open to Public Inspection No. 43115/1988 (hereinafter referred to as Japanese Patent O.P.I. Publication) wherein a change of a variable power considering aforesaid aberration correction is made by changing distances between aforesaid lens groups.
However, a zoom lens of a four-group type tends to be long in a total length because of an increase in the number of lens groups and consequently does not fit a compact camera. The cause for this longer total length is that the zoom lens tends to be of a structure of a reversed telescope for an entire zooming range, because the second lens group is given a greater power and a greater role of a variable power is assigned to the second lens group.
Further, because of the greater role of a variable power on the second lens group, the aberration variation caused by power variation in this lens group is great, which tends to limit the power and structure of following lens groups. Furthermore, because of aberration correction and other factors, each lens group needs to be driven independently and thereby the driving mechanism becomes complicated and the external dimensions of a lens barrel containing aforesaid driving mechanism also tends to be greater, which is a disadvantage.
In the case of a zoom lens for use with a lens shutter, it is not necessary to confirm focusing in a range-finder through a camera lens, which is different from a zoom lens for use in a conventional single-lens reflex camera. Therefore, it is possible to consider power variation drive and focusing drive separately. Taking this into consideration in designing an optical system, therefore, it is possible to make a zoom lens with an extremely compact arrangement wherein the zoom lens consists of the first, second, third and fourth lens groups arranged in that sequence from the subject side, and at least the first, third and fourth lens groups are moved on an optical axis toward the subject side for power variations from the shorter focal distance to the longer one, and focusing is conducted by means of the second lens group, and further, for the power variation, the first and third lens groups are moved together.
Further, it has been found that focusing by means of the second lens group contributes to miniaturization of a total zoom lens, the power of the first lens group can be relatively small due to the short distance of the movement of the second lens group for focusing, and an arrangement of lens movement can be simple owing to the first and third lens groups which move together for power variation.
However, it has been found that a driving mechanism for a varifocal lens optical system composed of four lens groups, the first through the fourth lens groups arranged in that sequence from the subject side, especially a mechanism for driving accurately the second lens group positioned between the first lens group and the third lens group which are driven together, transmitting the driving force mechanically and independently, tends to be too complicated. This point proved to be an extremely serious problem from the viewpoint of design. Therefore, there is demand for development of a mechanism which is small and is capable of driving lens groups with a simple structure and of moving them quickly to their determined positions.